The IEEE 802.11ac standard specifies the downlink multi-user multiple-input-multiple-output (DL-MU-MIMO) technique. One of the first steps in utilizing DL-MU-MIMO is to form groups of client devices (also called users, stations, etc.), where a group consists of at least two client devices that can receive spatial streams transmitted by a wireless network device simultaneously. A client device may belong to more than one group.
Groups need to be formed in such a way that interference between spatial streams for each client device within the same group is low enough to successfully deliver the simultaneously transmitted very high throughput (VHT) aggregated MAC protocol data units (A-MPDUs). At the same time, the scheduling of MU-MIMO burst needs to be done in such a way that the medium is used fairly.
According to the standard, the length of the frames (e.g., airtime) to all client devices within the same MU-MIMO burst has to be the same. When the frames have different airtime, padding is used to make sure that the MU-MIMO MPDU for all client devices are aligned to the same physical layer (PHY) symbol boundaries.